When aircraft are operating on the airport surface, it has been observed that reception of automatic dependent surveillance-broadcast (ADS-B) squitters from aircraft that are within line-of-sight of the ADS-B receiver is often not reliable. A closer examination of structures in the vicinity of the airport has revealed that multipath reflections from these structures are the likely explanation for the missed reception of the ADS-B squitters. When the aircraft is in the air or moving at take-off or landing speeds, the effect of the multipath reflection is inconsequential and may result in the loss of a single squitter. However, when the aircraft is stationary or moving slowly, the multipath reflections may hide the true ADS-B squitters.
The most common source of multipath in flat open areas such as airport runways is a ground reflection. However, the distances between aircraft operating on the airport surface are too short for the ground reflections to cause deep fades. Instead, reflections from buildings, other aircraft, etc are more likely to be the root cause of deep fades.
In order to understand the conditions under which multipath produces deep fading on the airport surface, we need to make use of the following physical relationships between the direct path and the reflected multipath.
The total received signal power, Pr, due to the direct and reflected signals is given byPR=PT[λ/4πd]2|1+R(θr)ejΔ|2 
where R(θr) is the reflection coefficient of the reflecting structure. The reflection coefficient has magnitude between 0 and 1 and phase between 0 and 180 degrees. Perfect cancellation between the direct path and the reflection occurs when the reflection coefficient R(θr) has magnitude unity and the phase of the reflection coefficient plus the relative phase shift Δ due to the difference in the distances traveled by the direct and reflected path add up to an odd multiple of 180 degrees.
The phase difference Δ and the difference in time of arrival (delay) τ between the direct and the reflected signals depends on the difference in the distances traveled, i.e.Δ=2π(s1+s2−d)/λ=2πfτ
where d, s1 and s2 are distances, λ=c/f is the wavelength, c is the speed of light and f=1090 MHz is the ADS-B carrier frequency.
In general, the amplitude and phase of the reflection coefficient depend on the angle of reflection, θr, the polarization of the signal, the dielectric and conducting properties of the ground, and the frequency, i.e.R(θr)=[sin θr−χ(θr)]/[sin θr+χ(θr)]
where
χ(θr)=[(∈r−cos2 θr)−j60σλ]1/2/[∈r−j60σλ], polarization perpendicular to reflecting surface
χ(θr)=[(∈r−cos2 θr)−j60σλ]1/2, polarization parallel to reflecting surface
where ∈r is the dielectric constant of the reflecting surface relative to unity in free space, σ is the conductivity of the reflecting surface in mhos/m and θr is the reflection angle.
An illustration of multipath propagation due to reflections from structures on the airport surface when one aircraft at location A1 on the airport surface is broadcasting ADS-B messages and another aircraft on location A2 is attempting to receive the messages is shown in FIG. 1.
The separation distance between the aircraft is d. Multipath interference occurs when a reflecting structure at location R is oriented relative to the two aircraft so that a signal transmitted by the aircraft in location A1 is reflected in the direction of the aircraft in location A2. The reflecting surface may be another aircraft or vehicle on the airport surface, or a building or any other fixed structure. The distance traveled by the reflected signal, s1+s2, is greater that the distance, d, traveled by the direct line-of-sight path between the two aircraft.
The time delay or difference in arrival time, τ, between the direct and reflected paths is given byτ=(s1+s2−d)/c  (1)
where c=3×108 m/s is the speed of light.
An ellipse with major axis (−a, a) and minor axis (−b, b) and focus points at A1 and A2 plots the locus of all possible locations where a reflecting surface R can produce a multipath reflection with time delay τ relative to the direct path along the major axis of the ellipse. The angle of arrival φ1 or φ2 of the multipath relative to the angle of arrival of the direct path can be anywhere between 0 degrees when the reflecting point is in front of the receiving antenna along the major axis of the ellipse to 90 degrees when it is off to the side and up to 180 degrees when it is behind the receiving system along the major axis of the ellipse.
The ADS-B squitter messages broadcast by each aircraft consist of sequences of 112 pulses of 500 nanoseconds' duration. Each pulse is broadcast within a 1 microsecond interval either in the first 500 nanoseconds or the second 500 nanoseconds slot. The slot occupied by each pulse indicates whether the information bit represented by the pulse is a “0” or a “1”. This is known as pulse position modulation.
The effect of multipath reflection on the reception of the sequence of pulses in an ADS-B message and the appropriate mitigation depend on the delay between the multipath reflection and the direct path as discussed in the paragraphs below.
When the difference in travel time, i.e., time delay, between the direct and reflected paths is small compared to the 500 nanosecond pulse duration, i.e., when τ<50 ns, the pulse that is received via the reflected path arrives practically within the same slot interval as the direct path pulse. In this case the two overlapping pulses may add up if the phase of the reflected pulse is the same as the phase of the direct path pulse in which case no harm is done. Or the two overlapping pulses may cancel if they have nearly equal amplitudes and they are 180 degrees out-of-phase. If the transmitting and receiving aircraft are stationary or moving slowly the entire sequence of pulses in an ADS-B message will suffer from destructive multipath cancellation resulting in a failure to receive the transmitted pulse, as illustrated in FIG. 2.
An idea of the extent of airport surface area over which ADS-B squitter message reception is vulnerable to missed squitter reception due to multipath with short delay offset may be obtained from the dimensions of the major axis and the minor axis of an ellipse corresponding to multipath reflections with 50 ns time delay. Table 1 below shows the dimensions of the 50 ns time delay ellipse as a function of the separation distance d between the transmitting aircraft and the receiving aircraft. Reflecting structures that are within an elliptical boundary with these dimensions will produce multipath reflections with short delays.
TABLE 150 ns Time Delay Ellipse DimensionsSeparationMajor AxisMinor AxisDistance d (m)Dimension 2a (m)Dimension 2b (m)10011556.820021578.930031596.0400415110.6500515123.4600615135.0700715145.7800815155.6900915165.010001015173.8
Like ground reflection multipath, short delay multipath can be mitigated by using more than one antenna and receiver (diversity) to receive the ADS-B messages. However, unlike ground multipath, the use of top mounted and bottom mounted antennas do not necessarily provide mitigation against short delay multipath due to reflections from structures to the side of the LOS path if the top and bottom mounted antennas have omni-directional patterns.
To understand how pulse cancellation due to short delay multipath reflections from side structures can be mitigated PR=PT[λ/4πd]2|1+R(θr)ejΔ|2 is re-written asPR=PT[λ/4πd]2G1(0)G2(0)|1+A(φ1,φ2,G1,G2)R(θr)ejΔ|2 
where A(φ1,φ2,G1,G2) is an amplitude factor which is proportional to the ratio of the transmitting and receiving antenna gains G1(φ1) and G2(φ2) in the direction of the reflected path to the antenna gains G1(0) and G2(0) in the direction of the direct path, i.e.
When the difference in travel time between the direct and reflected paths is in the order of or greater than the 500 nanosecond pulse duration, i.e., when τ>450 ns, the pulse that is received via the reflected path arrives in the next or later slot interval, thereby interfering with the correct decoding of the pulse position in subsequent pulse intervals as illustrated in FIG. 3. The delayed pulse may arrive in the unoccupied slot during the current or next pulse interval so that a pulse is received in both slot positions making it difficult to determine which slot position is the correct one. Or the delayed pulse may arrive in the same slot as the pulse transmitted in the next interval in which case cancellation may occur if the two pulses are 180 degrees out of phase or they may add up if they are in phase.
In any case, long delay multipath produces “intersymbol interference” that makes correct decoding of the sequence of ADS-B squitter message not possible.
Table 2 below shows the dimensions of the 450 ns time delay ellipse as a function of the separation distance d between the transmitting aircraft and the receiving aircraft. Reflecting structure outside an ellipse with these dimensions will produce multipath reflections with long delays.
TABLE 2450 ns Time Delay Ellipse DimensionsSeparationMajor AxisMinor AxisDistance d (m)Dimension 2a (m)Dimension 2b (m)100235212.6200335268.8300435315.0400535345.2500635391.4600735424.6700835455.2800935484.09001035511.210001135536.8Table 2 shows that multipath reflections with delays longer than 450 ns can be produced within an airport surface of not too big an extension. To get an idea of what the maximum delay of multipath reflections might be, Table 3 below shows the dimensions of a 2000 ns time delay ellipse (i.e., multipath reflections with delays up to 2000 ns possible at large airports).
TABLE 32000 ns Time Delay Ellipse DimensionsSeparationMajor AxisMinor AxisDistance d (m)Dimension 2a (m)Dimension 2b (m)100700692.8200800774.6300900848.64001000916.65001100979.860012001039.270013001095.480014001149.090015001200.0100016001249.0
Diversity reception with multiple antennas with different azimuth antenna patterns does not prevent incorrect decoding of ADS-B message bits when the delayed pulse arrives in a later unoccupied pulse position. In this case, a pulse is received in both possible pulse position slots making it impossible for the receiver to differentiate between the direct path pulse and the reflected path pulse, especially when the reflecting structure is a highly conducting metallic object so that both pulses have equal or nearly equal amplitudes. Traditionally, digital communications systems that suffer from destructive interference from multipath with delay that is longer than the duration of transmitted modulation symbols use adaptive equalization to continuously estimate and cancel the interference that one symbol inflicts on subsequent symbols (inter-symbol interference). The adaptive equalizers may be classified as linear equalizers if they employ a linear Finite Impulse Response (FIR) filter with adaptive “tap weights” or as nonlinear decision feedback equalizers if they employ a linear FIR as well as a feedback FIR filter to cancel the inter-symbol interference. The adaptation of the linear or nonlinear FIR filter tap weights may be classified as pilot or training data-assisted equalization if known data is interleaved with information bearing symbols or as blind equalization if no known data is interleaved. In order for the tap weights of the adaptive filters to converge to the optimum values that result in the cancellation of the inter-symbol interference, the equalizer must be trained for a sufficiently long period of time at the beginning of a transmission and then they must continue adapting to maintain the tap-weight settings at their optimum value if the channel multipath is slowly changing. Therefore adaptive equalizers are most effective when used in systems that transmit data continuously over a channel with multipath that changes in time or which transmit long bursts of data preceded by a sufficiently long training preamble.
In the case of ADS-B messages, the waveform has already been defined and cannot be modified to make it more suitable for use with the above mentioned adaptive equalization techniques. The ADS-B message is very short (112 bits long) and the preamble consists of only four pulses of known position (too short to train an equalizer). The entire ADS-B message duration including the preamble and extended squitter message is less than 128 microseconds. The short preamble makes the use of adaptive equalization techniques difficult at best.
Other types of receivers that are commonly used in multipath channels are RAKE receivers. A RAKE receiver combines (“rakes”) the multipath coherently to improve the signal-to-noise ratio. RAKE receivers are employed when the multipath does not cause inter-symbol interference. They are used in systems where gaps between symbols are interleaved to allow for the reception of the multipath.
In the case of the ADS-B message, the pulses in the message either have one pulse interval gap between them if they represent 00 or 11 bits, or the pulses are next to each other if they represent a 10-bit sequence or they have a two pulse gap if they represent a 01-bit sequence. When the transmitted pulses are adjacent each other, multipath that is 180 degrees out-of-phase with the direct path and which are delayed in the order of one pulse duration will cancel the next pulse. So a RAKE receiver will not be able to protect against this type of multipath interference and if there is no error correction, then this bit loss cannot be recovered.